The present invention relates generally to brush seals for sealing between a turbine rotor and a stationary component about the rotor and particularly relates further to a method of attachment for a raised platform sealing surface provided on the rotor for engagement by the tips of the bristles of the brush seal whereby rotor dynamic and thermal constraints on the use of brush seals in diaphragm packing areas of the steam turbine are overcome.
As explained in detail in U.S. Pat. No. 6,168,377B1 of common assignee, it is desirable to employ brush seals for sealing between the turbine rotor and the stationary rotor casing since brush seals have demonstrably improved sealing characteristics as compared with labyrinth type seals typically used at those seal locations. However, rotor dynamic and thermal constraints inhibit use of brush seals for example in the diaphragm packing area of a turbine. Localized rotor heating due to the friction caused by the bristles of the brush seal rubbing on the rotor surface magnifies the effects of rotor vibrations through the first and second critical speeds resulting in unacceptable radial rotor movement. For example, impulse design steam turbines typically operate above the rotors first bending critical frequency and often near the second bending critical frequency. This sustained rubbing and heat generated thereby can cause thermal bowing of the rotor or exacerbate an existing bowed condition of the rotor. Accordingly, there is a need to minimize or eliminate the rotor dynamic and thermal constraints to the use of brush seals in steam turbine rotors to enable widespread use of brush seals in turbine seal locations previously constrained from such use.
In accordance with the preferred embodiment of the present invention, there is provided an insert for securement on the rotor affording a raised annular continuous sealing surface in contact with the brush seal bristle tips to dissipate frictional heat without affecting rotor vibrational characteristics. Particularly, the rotor is provided with a locking device for locking a plurality of platform sealing segments about the rotor to form and thus locate the annular sealing surface in a position raised radially from the surface of the rotor. In this manner, heat is dissipated outwardly of the rotor surface with minimal or no thermal effect on the rotor. The locking device preferably includes at least one flange radially spaced from the rotor surface and having a plurality of axially extending teeth spaced circumferentially one from the other defining slots therebetween. A plurality of platform sealing segments are provided for securement to the locking device. Each platform segment has an arcuate sealing surface portion which, when the platform segments are secured to the locking device, form the continuous annular sealing surface engagable by the tips of the brush seal bristles. Each segment preferably includes a channel having in-turned flanges axially facing one another and having a plurality of circumferentially spaced teeth separated by slots.
To install the platform sealing segments, the teeth of the platform segments are aligned with the slots between the teeth of the locking device. When aligned, the segments are displaced radially inwardly to locate the teeth of the segments inwardly of the teeth of the locking device. It will be appreciated that when the segments are located in this manner, a continuous annular sealing surface is formed about the segments. To secure the segments to the locking device, the segments are rotated as a unit in a circumferential direction about the rotor axis to locate the platform sealing segment teeth radially inwardly of and in engagement with the overlying teeth of the locking device. The platform segments are assembled onto the rotor in a direction opposite to that of the rotor rotation. Stops at the end of the blind slots in the platform segments limit rotation of each segment. Additional locking of the segments to the rotor is not necessary but is desirable. By preferably locking one segment to the locking device, for example by employing one or more grub screws or by staking or welding the segment in place, circumferential rotation of the annular sealing platform relative to the rotor is precluded. It will be appreciated that the contact between the brush seal and the platform is located radially outwardly of the adjacent rotor surfaces thus dissipating the frictional heat outwardly of the rotor surface. Additionally, the platform sealing segment surfaces and locking device can be used on single and opposed flow steam turbines during retrofit.
In a preferred embodiment according to the present invention, there is provided a steam turbine comprising a rotor and a non-rotatable component about the rotor, a brush seal carried by the non-rotatable component, an arcuate sealing platform interposed between the brush seal and the rotor and having an arcuate seal surface, a locking device carried by the rotor, the locking device and the sealing platform having interengeagable elements responsive to circumferential movement of the locking device and the platform relative to one another for locking the platform against radial outward movement relative to the rotor and the brush seal being engeagable with the arcuate seal surface to seal between the rotor and the non-rotatable component.
In a further preferred embodiment according to the present invention, there is provided a steam turbine comprising a rotor having adjacent rotor stages each including a plurality of buckets and a diaphragm about the rotor straddled by the buckets, a brush seal carried by the diaphragm, an arcuate sealing platform interposed between the brush seal and the rotor, the bucket stages and the platform having an arcuate seal surface raised radially outwardly of a rotor surface between the bucket stages, a locking device carried by the rotor for locking the platform against radial outward movement relative to the rotor, the brush seal being engageable with the arcuate seal surface to seal between the rotor and the non-rotatable component radially outwardly of the rotor surface enabling dissipation of frictionally generated heat by contact between the brush seal and the platform with minimal, if any, thermal effect on the rotor.
In a further preferred embodiment according to the present invention, there is provided in a steam turbine having a rotor and a non-rotatable component about the rotor carrying a brush seal for sealing between the rotor and the non-rotatable component, a method of installing a sealing platform on the rotor to afford a sealing surface for the brush seal comprising the steps of (a) providing a locking device at a predetermined axial position on the rotor, (b) providing platform segments having sealing surface portions at circumferential locations about the rotor substantially in radial registration with the locking device and (c) securing the segments to the rotor by engaging the segments with the locking device and rotating the segments relative to the locking device enabling the sealing surface portions of the platform segments to form a continuous uninterrupted annular sealing surface about the rotor for engagement with the brush seal.